The present invention relates to an improved nickel base high temperature brazing and diffusion brazing alloy containing chromium as the principal addition and a method of non-pressure diffusion brazing using the brazing alloy of this invention.
Diffusion brazing relies upon solid-state diffusion or movement of atoms in both directions across the interface of the joint between the brazing alloy and the base metal. It necessarily follows that diffusion brazing alloys are formulated to complement the base material of the parts to be joined. Diffusion brazing alloys are thus generally nickel, iron or cobalt base alloys, depending upon the composition of the base materials. High strength superalloys have presented a particular problem for diffusion brazing because of their limited wettability. The problem is to formulate a diffusion brazing alloy which complements the base materials to form a good braze or bond having the requisite physical properties, and which may be brazed at temperatures low enough for commercial applications. Higher brazing temperatures adversely affect the physical properties of the joint.
Various braze temperature depressants, including boron, have been utilized in diffusion brazing alloys, however the alloys have not been satisfactory for certain high strength alloys, particularly nickel base gamma prime strengthened alloys utilized in non-weldable aircraft turbine vanes and the like. Improved shear strength and uniformity of hardness is also required for such applications.
Examples of prior art diffusion brazing or bonding alloys include U.S. Pat. Nos. 3,759,629 and 3,700,427 assigned to General Electric Company, which disclose nickel based diffusion brazing alloys for brazing superalloys. U.S. Pat. Nos. 3,678,570 and 4,038,041 disclose methods and compositions for diffusion bonding wherein a thin alloy foil or layer is applied between the metals to be joined, substantially increasing the cost of diffusion brazing. The prior art also includes coating compositions including yttrium, which is an important constituent of the diffusion brazing alloy of the present invention, however such compositions would not be suitable for diffusion brazing superalloys contemplated by the present invention. U.S. Pat. No. 3,676,085 discloses a cobalt base coating for superalloys including yttrium. The disclosures of the above referenced prior art patents are incorporated herein by reference.